biblatex: When is the content of a field available

Question

I am currently adding the possibility to my style biblatex-fiwi to add an alternative title to films. This is in itself is not complicated. But I also want a mechanism which handles different scripts. The idea is that, depending on the script, a bibmacro will be created which can later be defined to load a specific supports which supports the needed script.

I added the two fields alternatetitle and alternatetitlescript. For this, the following lines must be added to a .dbx-File, for example biblatex-dm.cfg:

\DeclareDatamodelFields[type=field, datatype=literal]{alternatetitle}
\DeclareDatamodelFields[type=field, datatype=literal]{alternatetitlescript}
\DeclareDatamodelEntryfields[movie,misc,video]{alternatetitle, alternatetitlescript}

The following MWE example is a bit complicated:

\RequirePackage{filecontents} \begin{filecontents}{\jobname.bib}
  @movie{themovie,
        Address = {USA},
        Alternatetitle = {Аэлита},  
        Alternatetitlescript = {cyrillic},  
        Director = {Mad, Max},
        Title = {The Title},
        Year = {1996}}
\end{filecontents} 
\documentclass{article} 
\usepackage[style=fiwi]{biblatex} 
\usepackage{ragged2e}
\renewbibmacro*{filmtitle}{\iffieldundef{maintitle}
  {\printtext{\printfield[film]{title}}}
  {\printfield[film]{maintitle}\newunit}% 
\iffieldundef{alternatetitle}
  {}
  {\usebibmacro{bbx:cite-alternatetitle}} 
\iffieldundef{subtitle}%
  {}% 
  {\setunit{}% 
\printtext{\addspace\printfield{subtitle}}}%
\iffieldundef{volume}% 
  {}% 
  {\printfield[season]{volume}}%
\iffieldundef{number}% 
  {}%
  {\addcomma\addspace\printfield[episode]{number}}%
\iffieldundef{maintitle}% 
  {}%
  {\addcolon\addspace\printfield[film]{title}}% 
\ifpunctmark{!}{\unspace.\newunit}{\adddot}}%

\newbibmacro*{bbx:cite-alternatetitle}{
\iffieldundef{alternatetitlescript}
  {\addslash\printfield{alternatetitle}}
  {\addslash\ifbibmacroundef{altscript:\strfield{alternatetitlescript}-font}
  {\textsc{\thefield{alternatetitle}}}{\usebibmacro{altscript:\strfield{alternatetitlescript}-font}%
    {\thefield{alternatetitle}}} }}

\addbibresource{\jobname.bib} 
\usepackage{fontspec} 
\setmainfont{TeX Gyre Pagella}
\newbibmacro*{altscript:cyrillic-font}[1]{\newfontfamily\cyrfont[Extension = .ttf,]{Linux Libertine}{\cyrfont{\textsc{#1}}}} 

\begin{document} 
\citefilm{themovie} 
\printbibliography 
\end{document}

The idea is that a macro called altscript:<alternatetitlescript>-fontis created which can the be defined in the document. The problem is that the content of alternatetitlescriptis not resolved. This is even more strange, as the test whether alternatetitlescriptis empty or not, actually works. But later when it should be become part of the macro name, the content alternatetitlescriptseems not available.

Answer 1

The unstarred version of \usebibmacro sanitises the macro name with a \detokenize. Since we want the \strfield{alternatetitlescript} to be expanded, we need to work around that. The starred version \usebibmacro* does not do any sanitising on the macro name and so

\usebibmacro*{altscript:\strfield{alternatetitlescript}-font}{\thefield{alterna‌​tetitle}}

works as expected.